Role of ethanol-inducible cytochrome P450 (P450IIE1) in catalysing the free radical activation of aliphatic alcohols

Incubation of rat liver microsomes with 1-propanol and 1-butanol in the presence of NADPH and of the spin trapping agent 4-pyridyl-1-oxide-t-butyl nitrone (4-POBN) allowed the detection of free radical intermediates tentatively identified as 1-hydroxypropyl and 1-hydroxybutyl radical, respectively. Microsomes isolated from rats treated chronically with ethanol (EtOH) or with the combination of starvation and acetone treatment (SA), exhibited a two-fold increase in the ESR signal intensity as compared to untreated controls, whereas no increase was observed in phenobarbital-induced (PB) microsomes. Consistently, in reconstituted membrane vesicles, ethanol-inducible cytochrome P450IIE1 was twice as active as phenobarbital-inducible P450IIB1 in producing 1-butanol free radicals. In the microsomal preparations from EtOH and SA pretreated rats the addition of antibodies against cytochrome P450IIE1, but not of preimmune IgGs, lowered the ESR signal of 1-butanol radicals by more than 50%. The same antibodies decreased the free radical production by untreated microsomes by 35-40%, but were ineffective on microsomes from PB-treated animals. This indicated that cytochrome P450IIE1 is the major enzyme responsible for the free radical activation of alcohols in control and ethanol-fed rats. The generation of 1-hydroxybutyl radicals by EtOH microsomes was inhibited by 40, 48 and 68%, respectively, by the addition of isoniazid, tryptamine and octylamine, compounds known to specifically affect the NADPH oxidase activity of this isoenzyme. This effect was not due to the scavenging of the alcohol radical since none of these compounds affected the ESR signals originated from 1-butanol in a xanthine-xanthine oxidase system. When added to reconstituted membrane vesicles isoniazid, tryptamine and octylamine also decreased 1-butanol radical formation by P450IIE1 by 54, 38 and 66%, respectively. Such an inhibition corresponded to the effect exerted by the same compounds on O₂ ^- release from P450IIE1 containing vesicles. These results indicate that the capacity of cytochrome P450IIE1 to reduce oxygen is related to its ability to generate alcohol free radicals and suggest that ferric cytochrome P450-oxygen complex might act as oxidizing species toward alcohols.